The present invention relates to the surface treatment of thermoplastic substrates and more particularly to an improved solvent system therefor.
The interaction of solvent systems (liquid or vapor at room temperature or at elevated temperature) with plastic materials for their cleaning, polishing, or other apparent superficial property enhancement is well documented in the art. U.S. Pat. No. 2,294,479 combines the good solvency power of ketones, esters, ethers, aromatics, or alcohols for cellulose and vinyl coatings with the non-solvent (non-explosive) characteristics of chlorinated solvents for elevated temperature, atmospheric vapor-phase polishing or surface smoothing of a cured lacquer coating system. For example, cellulose lacquer paint films were surface polished with an azeotropic mixture of 29 weight parts methyl ethyl ketone and 71 weight parts carbon tetrachloride at an elevated temperature of 73.8.degree. C. U.S. Pat. No. 3,020,661 proposes to polish the surface of cellulosic polymers as well as remove extraneous mold flash by utilizing a combination of ketone solvents and chlorinated non-solvents at elevated temperature and at atmospheric pressure. This proposal notes the importance in maintaining the temperature of the solvent mixture above its boiling point but below the melting point of the cellulosic polymer and for maintaining a contact time sufficient for leveling out the surface of the polymer while simultaneously removing the mold flash from the plastic part.
U.S. Pat. No. 3,807,054 also is directed to an atmospheric pressure vapor phase plastic surface polishing process wherein polyethyelene parts are treated with a mixture of chlorinated hydrocarbon solvents and cellulose acetate. U.S. Pat. No. 4,302,418 utilizes high temperature conditioning of polymethylpentene plastic materials so that a chlorinated solvent will interact with the plastic material and cause flow-out and improved surface appearance qualities. The chlorinated solvent of choice is defined to be a non-solvent for the plastic substrate at lower temperatures. U.S. Pat. No. 3,684,553 utilizes a vacuum process for controlling vapor pressure of a solvent-plastic smoothing composition while U.S. Pat. No. 3,473,960 proposes to totally immerse the plastic part in a solvent in order to achieve polymer-flow or surface smoothing.
The foregoing art does not recognize certain fundamental critical relationships regulating the interaction of solvents (liquid or vapor at low and elevated temperature) and polymeric materials. The classification of a solvent as being a "solvent" or a "non-solvent" for a particular plastic material is inadequate for properly determining whether a particular solvent or solvent mixture will display a deleterious effect on the plastic material regardless of the phase of the solvent (gas or liquid) or the contact temperature conditions. It is important to recognize that each plastic material is different in chemical structure and will not respond favorably to just any generalized class of solvent systems which the art teaches to be useful in surface smoothing processes. For example, polyethylene has been shown to be capable of being surface-finished with chlorinated solvents, but it is documented that polyethylene parts also undergo brittle fracture when contacted with chlorinated solvents, alcohols, esters, phenols, aromatic hydrocarbons, and aliphatic hydrocarbons. Polyethylene is known to fail rapidly under load conditions in the presence of non-solvents such as, for example, hexane or xylene at room temperature (H. R. Brown, "Polymer", Vol. 19, 1186, 1978). Polycarbonate rapidly loses its original tensile strength when exposed to amines, ketones, aldehydes, chlorinated hydrocarbons, low molecular weight esters and ethers, and aromatic hydrocarbons. Nylons are known to be resistent to most solvents, but degrade rapidly in the presence of phenols and aqueous solutions of inorganic acids. Polysulfones degrade in the presence of amines, ketones, aldehydes, chlorinated solvents, ethers, aromatic hydrocarbons, phenols, amides, and nitriles. Polyphenylene oxides degrade in the presence of amines, ketones, aldehydes, chlorinated hydrocarbons, esters, ethers, certain aliphatic and aromatic hydrocarbons, nitriles, and phenols (D. G. Brady and H. W. Hill, "Modern Plastics", 60, May, 1974). Both polycarbonate and polyacrylate (polymethacrylate) plastics are extremely sensitive to different classes of solvent interactions under load conditions (V. D. McGinniss, "Organic Coatings and Plastic Chemistry", Vol. 39,529,1978).
The prior art uniformly discloses solvent-polymer interactions which frequently lead to immediate or long-term degradation of the ultimate structural polymeric properties. The present invention is a significant step forward in the art in its ability to select practical solvent systems for surface finishing of a wide variety of plastic materials without damaging the original polymer structure or making it unsuitable in its original use for which it was intended. Immediate as well as long-term degradation effects of brief solvent-exposure processing operations are minimized or eliminated in accordance with the present invention.